Closed circuit grinding



July 28, 1942. N. A. MaCLEoD CLOSED CIRCUIT GRINDING Filed Oct. 10, 1939 Patented July 28, 1942 CLOSED CIRCUIT GRINDING Norman A; MacLeod, Britannia Beach, British Columbia, Canada,v assignor of. one-half to Alexander C; Munro, Britannia Beach, British Columbia, Canada Application October 10, 1939,.SerialNo. 298,774

6 Claims;

This invention relates to closed circuit wet grinding systems and pertains particularlyfto a closed circuit grinding system adapted todeliver a coarse product to acontinuouslyr operating grinding mill such as a ball mill anditoproduce a classified ground product in a continuous process.

One of the particular objects of thezinvention is to provide a closed circuit wet grinding, system which is characterized by the establishment of a predetermined rate of circulating load to give the maximum efficiency to the ball millor other grinding apparatus.

A further object of the invention is to provide a closed circuit Wet grinding system in which the moving parts are reduced to an essential minimum, particularly in the vicinity of theclassifler portion of the system, whereby minimum interference with true settling in the classifier element is had, with a consequent increase inclassication eiciency.

A further object of the invention is to provide a self-regulated Wet grinding system employing a pump device to remove the discharge of 'settled material from a settling device to'a grinding device, wherein a steady overflow of fines is obtained from the settling device independent' of surges inherent in the operation of a pumping system under ordinary conditions.

A further object ofthe invention is to provide a wet grinding system in'which fluctuations or surging in the system, as Well as'changes indilution ofthe pulp passingV through the system, have but a minor effect upon the operating conditions,- whereby maximum efficiency of both grinding and classification are concomitantly secured.

The grinding system of the present invention involves the use of a settling tank-adapted to re'- ceive the Wet discharge of a wet grinding mill, preferably together with the original feed, and to separate the material delivered thereto into a, relatively fine classied product and a relatively coarse reject product, discharge meansV receiving the reject product together with water from this classifying tank and including means for elevating the same to a level above the lWet grinding mill, a second settling tank adapted to receive ithe elevated reject product and separatethe same into two products, the one comprising4 principally the settled solid-rejectvmaterial4 Whicliis introduced into the Wet grinding mill and the other comprising the carrier water which isthen discharged together with the above mentioned ball mill discharge and original feed into-the rst mentioned classifying tanks Further objects and advantages of the invention will be brought out in the ensuing description .of preferred embodiments of the invention, as nillustrated in the accompanying drawing, or will be apparent therefrom. With reference to the-drawing:

Fig; 1'. is a partly schematic elevation of one form .of ,the present closed circuit grinding system; and y Fig. 2 is a fragmentary detail illustrating a modification of the manner of introducing the feedto the system.

Referring to Fig. 1, a Wet mill is indicated at I (itbeing appreciated that the term wet mill is, employed inageneral sense, to include the conventional ball mill, a rod mill, a tube mill, a chaser mill, or the like, operating with a carrier liquidfsuch as water). The Wet mill may comprise a ball mill, as shown, which is provided with conventional trunnions 2, an inlet 3 passing through one trunnion and a discharge 4 leading from the other trunnion. A classifying tank` is indicated at 5, (the term classifying tank being usedfin a general sense to include various types of settling devices, such as a settling cone, a spitzkasten, or a settlingr tank member provided Withsubmerged rakes or the like, as will be apparent to those skilled in the art). The settling-,tank 5 is provided with a surge box 6,

.a bafe I and an overflow Weir 8, and the discharge pipe 4 from the ball mill I and the original feed pipe 9`are directed into the surge box 6. The settling tank 5 is provided atV the bottom with'a discharge line I I, preferably provided with a valve I2 and'communicating to the inlet of a pump member I3 provided with a discharge line I4',Yva1ved as at I5, which discharges into a surgebox I6 of a second classifying tank Il 1ocated at a level above the ball mill I. The classifying tank Il is provided With an overflow Weir I anda discharge lineV I9 communicating with the surge box I5A of the classifier tank 5. The settlingssfrom the classifying tank Il are discharged'from the bottom thereof as through a valveddischarge line 2 I, communicating with the inlet' line 3 above mentioned, to the ball'mill I.

Insthe use of the system, Where a material such as-'an ore, together With a carrier liquid such as water, isintroducedat 9 to thesurge boX 6 of theetank 5; the portion thereof which is of sumcient'neness that no grinding is required is overilowedfrom' the tank 5 at the overflow 8 and the coarse or sands material is discharged through theiline II to the pump I3 at a rate slightly-greater'than the desired rate of feed to the ball mill. This withdrawn material is pumped up by pump I3 to the second classifying tank I1 from which the sands settle and flow through the pipes 2| and 3 to the ball mill at the desired rate of feed, that is, the discharge from the tank I'I is choked off at the valved line 2| so that a definite volume of water and material to be ground is supplied to the mill, a quantity less than that handled by the pump I3. 'I'he excess material handled by the pump I3 is discharged from the tank I`| as at I8 through the line I9 into the tank 5, to provide a continual return to the latter tank, wherefore any uctuations which may occur in the pump operation, within the margins allowed for by the capacities of the various elements of the apparatus, will make no fiuctuations in the quantity of the material supplied to the ball mill or in the rate of fluid overfiow from the classifying tank 5, so that maximum efficiency of operation of the ball mill is secured together with maximum classifying efficiency of the tank 5. The only variant in the system is the original feed, which is normally maintained within close limits in any operation.

Through the use of a controlled discharge from the tank I'I to the ball mill I, at the valved line 2|, a positive rate of feed can be maintained so that the mill can be operated at a predetermined rate of circulating load such as to give maximum efficiency for the system, and the sands discharge from the classifying tank Il to the ball mill I from a higher level is particularly advantageous from the standpoint of enabling a direct feed to the mill through the trunnion, avoiding the use of a scoop and scoop box; the surging of the system due t variations in the pump performance are substantially eliminated as far as the effect thereon upon the classifying operation of the tank is concerned in view of the fact that any excesses resulting from such variations are closed-circuited through the overflow from the tank I'I to the tank 5, and these surgings are balanced in the closed circuit thus reducing to a minimum any disturbing effect on the classifier overflow at the tank 5.

The system presents the possibility of employing an additional control of the mill feed by the addition of water or other carrier liquid at the tank I'I, as through a valved line 22, so that extra settling. could be obtained if the tank were made sufliciently large by increasing the dilution of the pulp at this point, which results in increasing the percentage of solids in the feed to the ball mill over that in the material supplied to the pump from the tank 5, and at the same time the volume of the classifier overflow from the tank 5 would be increased.

The pump I3 is shown as being of the centrifugal type, which, when rubber lined, will give admirable service, but it will be appreciated that any other form of elevating means may be employed to carry the sands and water from the bottom of the tank 5 into the tank I'I, Without departing from the scope of this invention. It will be noted that the tank I'I is shown as being somewhat smaller than the tank 5, for the reason that the actual classification of material is accomplished at the tank 5 and the tank I'I may be relied upon merely to afford adequate settling of the sands for delivery thereof to the ball mill. Variations of the relative capacities of the several elements of the construction will occur to those the hereindisclosed relative proportions are exemplary only.

In Fig. 2 I have illustrated a modification of the system in which the original feed is introduced into the system at the position of the tank I'I, instead of at the position of the tank 5 as shown in Fig. 1. Referring to this figure, a tank I1' is illustrated, provided with a sands discharge 2 I a surge box I6', a valve inlet I5' communicating with a pump such as shown at I3 in Fig. l, and an overflow I8. A feed supply line 9 is shown, for introducing the feed to the surge box I6', according to this embodiment, and a liquid supply line 22 may also be provided if desired.

In the operation of a system provided with a separating tank of the type shown in Fig. 2 in vwhich the feed is introduced at the upper separating tank, the first separation takes place in the tank II, and the sands material is withdrawn through the discharge line 2|' to the ball mill at the desired rate. The overflow from the tank I1 is then carried as through a conduit I 9 to the surge box of the lower separating tank together with the discharge from the ball mill, and the overflow from this lower separating tank may be considered to be the finished product. The san-ds withdrawn from the lower separating tank and elevated to the surge box of the upper separating tank will thus comprise a relatively smaller quantity of material with a given ball mill throughput than is the case with the system shown in Fig. l, but the sum of the materials admitted to the surge box I6 of the upper separating tank I1', i. e., the material supplied at I5 plus the material supplied at 9', will be in excess of the sands material withdrawn at 2| and supplied to the mill, wherefore the two substantially separate sands and overflow circuits are established in a manner comparable to that described in connection with Fig. 1, and the resultant elimination of surging effects is obtained.

I claim:

1. The method of grinding ores or the like which comprises: feeding the ore into a body of liquid in a gravity type 0f separating tank to separate the same into a fines product and a sands product in the presence of such liquid; discharging the fines product together with a portion of such liquid from said separating tank; separately discharging the sands product together with another portion of such liquid from the bottom of said separating tank under gravity fiow; elevating said sands product and liquid; discharging said elevated sands product and liquid into a second separating tank to separate the same into a sands product and a liquidcontaining fines overflow product; discharging said overflow product into said rst separating tank together with said ore feed; discharging said last-mentioned sands product under controlled gravity flow from said second separating tank into a wet grinding device; and discharging the ground product from such wet grinding device into said first separating tank together with said ore feed and said overflow product from said second separating tank.

2. The method set forth in claim 1, in which the flow of sands product from the second separating tank to the wet grinding device is established at a rate of flow less than the rate of discharge of sands product from the first separating tank, wherefore a portion of the sands .product and water from the first separating tank is continuously returned to the rst separating tank from the second separating tank.

3. The method set forth in claim 1, in which additional liquid is added to the circuit at the position of the second separating tank, whereby the pulp dilution is increased at this point in the circuit, and in which the flow of sands product from the second separating tank to the wet grinding device is established at a value less than the rate of discharge of sands product from the rst separating tank, wherefore a portion of the sands product from the first separating tank is continuously returned to the rst separating tank.

4. A closed circuit wet grinding system which comprises: a wet grinding element; a settling tank positioned to receive the discharge of said grinding element and provided with a feed inlet, a nes overflow at the upper portion thereof, and a sands discharge opening at the bottom thereof; conduit means communicating with said discharge opening and providing a gravity flow passage leading downward therefrom; elevating means in communication with said conduit means and positioned to receive the sands by gravity now from said separating tank and elevate the same to a level above said wet grinding element; a second settling tank positioned to receive the elevated sands and provided with a bottom sands discharge and a top fines overflow; conduit means for conveying separated sands from said second settling tank to said wet grinding element; conduit means for conveying nes from said second settling tank to said rst settling tank adjacent the position of said feed inlet,

and valve means in said first-mentioned conduit means for restricting the ow of sands to said wet grinding element.

5. A closed circuit wet grinding system which comprises: a wet grinding element; a settling tank disposed at a level below said wet grinding element and provided with an inlet, a bottom sands discharge, and a top overflow; a second settling tank disposed above said Wet grinding element and provided with an inlet, a :bottom sands discharge, and a top overow; elevating means disposed below said rst settling tank; conduit means connecting the sands discharge of the first settling tank with said elevating means for gravity flow thereto, conduit means connecting said elevating means with the inlet of said second settling tank, conduit means connecting the sands discharge of said second settling tank to the inlet side of said grinding element for gravity iiow thereto and conduit means connecting the discharge side of said grinding element to the inlet of said first settling tank, to define a closed sands circuit; and conduit means connecting the overflow of said second settling tank to the inlet of said rst settling tank, to complete an overow circuit through said first settling tank, elevating means, and the second settling tank.

6. A closed circuit wet grinding system as set forth in claim 5, and comprising in addition, valve means in the portion of said rst mentioned conduit means which connects the sands discharge of said second settling tank to said grinding element.v

NORMAN A. MACLEOD. 

